An integrated circuit (IC) package provides a number of functions for an IC that can be critical to its operation. These include (1) delivering power to the IC, (2) transferring information into and out of the IC, (3) dissipating heat, and (4) protecting the IC from physical and/or environmental damage. To the extent that the package is damaged or defective, one or more of these functions can be impacted. Depending on the degree of impact, the yield, functionality, and/or reliability of the IC can be affected.
Shown in FIG. 1 is a cross sectional view of ball grid array (BGA) package 100 that includes a BGA substrate 102 and an IC 104. The IC 104 is electrically connected to the BGA substrate 102 by way of controlled collapse chip connection (C4) solder interconnects 106. Underfill material 107 occupies regions between the BGA substrate 102 and the IC 104 and forms fillets 108 along the sidewalls of the IC 104. The C4 solder interconnects 106, underfill material 107, and fillets 108, in combination, rigidly attach the IC 104 to the BGA substrate 102.
The BGA substrate 102 electrically connects the C4 interconnects 106 to the BGA interconnects 110 by way of conductive traces, vias, and plated-thru-holes (not shown) in the BGA substrate. The BGA interconnects 110 connect to external circuitry 112, for example a motherboard. Here, the BGA substrate comprises three regions—a middle core region 102B and upper and lower build-up layer regions 102A and 102C, respectively.
Turning to FIG. 2, a more detailed cross-sectional view 200 of portions of BGA substrate 102 that includes build-up layer regions and core regions is shown. The build-up layer regions 102A and 102C each comprise a plurality of polymer build-up layers 204 and conductive build-up layers 202. The conductive build-up layers (typically copper) form conductive paths within the build-up layer regions that are interconnected by way of vias (not shown) disposed in intervening polymer build-up layers 204. The core 102B is a reinforced core that comprises glass fibers 208A/208B embedded in a polymer resin material 206, such as bismaleimide triazine (BT) epoxy. Plated-thru-holes thru the core region (not shown) connect conductive leads in the build-up layer region 102A with conductive leads in the build-up layer region 102C. The composite substrate 102 forms a relatively rigid and inflexible structure.
To the extent that the packaged IC experiences changes in temperature, for example during assembly or operation, the IC 104 and the package substrate 102 will expand and contract. The IC and substrate expansion and contraction typically occurs by different amounts because the IC and substrate comprise different materials that have different coefficients of thermal expansion (CTE). Because the IC 104 is rigidly attached to the package substrate 102, and the CTEs of the two do not match (e.g. the IC typically has a CTE of approximately 2-3 and the package substrate typically has a CTE of approximately 17-20), the expansion/contraction of the IC 104 and substrate 102 can cause bending of the package 100. This can impart strains on the BGA interconnects 110 which can over time produce BGA interconnect fatigue and failure.